In terms of this water-carrying analogy, the frequency of going up
and down the ladder should be nearly constant (that, is like our 60 cycles per
second electrical frequency). So, when more water is needed, the amount that
each person carries up the ladder must get bigger (since they are not allowed
to go faster or slower). Well, if this water gets too heavy, either the ladder
might break, or the person might get too tired to carry it. We could argue that
if the ladder breaks, that is like the outage of a transmission line that
either sags or breaks under the stress of too much current. There are devices
called relays in an electrical system that are supposed to sense when the load
is too much and send a signal to a “circuit breaker” to remove the line from
service (like removing the set of three ladders). If the person gets too tired,
we could again stretch this analogy to say that this is like not having enough
reactive power (resulting in low
1 Another analogy
that says that reactive power is the “foam on the beer” is fairly good here
because the space in the glass is taken up by the useless foam - leaving less
room for the “real” beer.
voltage). In the
extreme case, the person might “collapse” under the weight of the water that
the person is being asked to carry. If it happens to one person, it will
probably happen to many of them. In the electrical system this could be
considered a “voltage collapse”. While there are “undervoltage relays,” there
are no relays in the system to directly sense the problem that the voltage is
about to collapse.
Remember, the
people going up and down the ladders do not absorb or produce energy over a
complete cycle and are therefore analogous to reactive power. It is the water
going up the ladder to fill the tank that absorbs real power that must be paid
for. But, the real power cannot be delivered without the reactive power. And,
if there is not enough reactive power (like with people going up and down the
ladders), the real power delivery will eventually fail.
In summary, a
voltage collapse occurs when the system is trying to serve more load than the
voltage can support. A simulation has been prepared to illustrate voltage
collapse by simply using a system with an Eastern generator and customer load,
a Western generator and customer load, and East to West transmission lines. In
the simulation, the Eastern generator has a constrained supply of reactive
power and progressive line outages for unspecified reasons lead to a voltage
collapse even when reactive power supply is ample at the Western generator. The
results of the simulation are available in Power Point slides. (If you are
connected to the Internet, click here to view the slides.)
In contrast to all
of this, you could route a hose up the side of the water tower and simply turn
on the water and let the water flow in the hose to fill up the tank. The water
pressure is like voltage, and the water flow is like current. This type of
system would be a direct current (DC) system and would not involve reactive
power at all. However, the concept of voltage collapse is not unique to AC
systems. A simple DC system consisting of a battery serving light bulbs can be
used to illustrate how too much load on a system can lead to a condition where
voltages drop to a critical point where “adding more load” results in less
power transmission - a form of voltage collapse. (If you are connected to the
Internet, click here to view Power Point slides illustrating DC voltage
collapse.)
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